This application claims priority to an application entitled xe2x80x9cTransmit Antenna Diversity Apparatus and Method for Base Station in a CDMA Mobile Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Dec. 21, 2000 and assigned Serial No. 2000-79713, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a communication apparatus and method in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to a forward transmit antenna diversity apparatus and method in a CDMA mobile communication system.
2. Description of the Related Art
An existing CDMA mobile communication system that mainly supports voice service, has been developed into a future (CDMA mobile communication system which provides high-speed data service as well as voice service. The future CDMA mobile communication system supports voice, moving image and Internet search services. In the mobile communication system, communication links existing between a base station and a mobile station are classified into a forward link for transmitting a signal from the base station to the mobile station, and a reverse link for transmitting a signal from the mobile station to the base station.
The mobile communication system must resolve a fading problem in order to transmit high-speed data. The fading causes a reduction in the amplitude of a received signal from several dB to several tens dB. In order to solve the fading problem, a variety of diversity techniques are used.
One of the techniques used in the CDMA system employs a Rake receiver, which receives a signal on a diversity basis using delay spread of a channel and the Rake receiver supports a reception diversity technique for receiving a multi-path signal. However, this diversity technique is disadvantageous in that it is not operable when the delay spread is low in level.
Also, a time diversity technique utilizing interleaving and coding is used in a Doppler spread channel. However, this technique is not effective in a low-speed Doppler spread channel. It is possible, though, to effectually solve the fading problem using a space diversity technique, in an indoor channel with a low Doppler spread level and a pedestrian channel, a low-speed Doppler channel.
The space diversity technique uses two or more antennas. In this technique, even though a signal transmitted through one antenna is attenuated due to the fading, it is possible to compensate for the attenuation using a signal transmitted through the other antennas. The space antenna diversity technique is divided into a reception antenna diversity using a plurality of reception antennas and a transmit (transmission) antenna diversity using a plurality of transmission antennas. It is hard to install the reception antenna diversity in the mobile station in light of its size and cost. Thus, the use of the transmit antenna diversity for the base station is recommended.
The transmit antenna diversity includes a xe2x80x9cclosed loop transmit diversityxe2x80x9d transmitting a signal based on forward channel information fed back from the mobile station, and an xe2x80x9copen loop transmit diversityxe2x80x9d receiving no feedback information from the mobile station. In the closed loop transmit diversity scheme, the base station applies weights to transmission signals of the respective transmission antennas based on the channel information measured and fed back by the mobile station to maximize a signal-to-noise ratio (SNR) of an antenna at the mobile station. In the open loop transmit diversity scheme, the base station transmits the same signal through two quadrature (or orthogonal) paths without using the feedback information. The quadrature paths can be provided by time division, frequency division or code division.
FIG. 1 illustrates a structure of a base station transmitter using an open loop transmit diversity scheme according to the prior art. Referring to FIG. 1, an input bit stream is encoded by a channel encoder 101, and an output sequence of the channel encoder 101 is mapped into an M-ary symbol by an M-ary symbol modulator 102. The M-ary symbol modulator 102 serves as a QPSK (Quadrature Phase Shift Keying), 8-PSK (8-ary Phase Shift Keying) or 16-QAM (16-ary Quadrature Amplitude Modulation) modulator according to its data rate, and its modulation mode can be changed in a physical layer packet unit where the data rate can be changed. I and Q sequences of the M-ary symbol output from the M-ary symbol modulator 102 are modulated into two different complex symbols by an STTD/STS (Space-Time Transmit Diversity/Space Time Spreader) modulator 103. A detailed description of the STTD/STS modulator 103 will be made with reference to FIGS. 4 and 5. Walsh cover parts 104 and 105 orthogonally spread their input symbols using a Walsh orthogonal code WNi assigned to the mobile station. A detailed structure of the Walsh cover parts 104 and 105 is illustrated in FIG. 2. The two complex symbols spread by the Walsh cover parts 104 and 105 are subject to complex spreading by their associated complex spreaders 106 and 107, respectively. An internal operation of the complex spreaders 106 and 107 is illustrated in FIG. 3. The output signals of the complex spreaders 106 and 107 are shifted to RF (Radio Frequency) band signals by associated RF parts 108 and 109, and then radiated through first and second antennas ANT1 and ANT2.
FIG. 2 illustrates a detailed structure of the Walsh cover parts 104 and 105 illustrated in FIG. 1. Each Walsh cover part 104 and 105 spreads its input complex symbol to a transmission bandwidth, using a Walsh code assigned to a transmission channel. FIG. 3 illustrates an internal operation of the complex spreaders 106 and 107 shown in FIG. 1. Each of the complex spreaders 106 and 107 complex-spreads its input complex signal into an I-channel (or I-arm) signal and a Q-channel (or Q-arm) signal, using a spreading sequence comprised of an I-channel spreading sequence PNI and a Q-channel spreading sequence PNQ.
FIG. 4 illustrates an internal operation of the STTD/STS modulator 103 of FIG. 1 when it operates in an STTD (Space-Time Transmit Diversity) mode. In the STTD mode, the STTD/STS modulator 103 operates as shown in Table 1.
In Table 1, S0 and S1 represent complex symbols, and are represented by
S0=Si0+jSq0
S1=Si1+jSq1
If symbols S0 and S1 are input to the STTD modulator 103 at a specific time t and a time t+T, respectively, then the STTD modulator 103 outputs the symbol S0 for the first antenna ANTI and a minus conjugate of the symbol S1 for the second antenna ANT2 at the time t, and outputs the symbol S1 for the first antenna ANT1 and a conjugate of the symbol S0 for the second antenna ANT2 at the time t+T.
FIG. 5 illustrates an internal operation of the STTD/STS modulator 103 of FIG. 1 when it operates in the STS (Space Time Spreader) mode. Referring to FIG. 5, a serial-to-parallel (S/P) converter 501 converts each of its input complex symbols comprised of an I-phase symbol and a Q-phase symbol into two xc2xd-rate complex symbols comprised of an I-phase symbol and a Q-phase symbol. The two complex symbols I1/Q1 and I2/Q2 are provided to symbol repeaters 511-518, where they are repeated. For example, the symbol I1 is input to the symbol repeaters 511 and 515. The symbol repeater 511 (++) repeats the input symbol I1, while the symbol repeater 515 (+xe2x88x92) repeats the input symbol I1. The outputs of the symbol repeaters 511-518 are provided to four summers 519-522 and then converted to two complex symbols. Herein, the STTD/STS modulator will be referred to as a xe2x80x9cdiversity modulatorxe2x80x9d for simplicity.
FIG. 6 illustrates a structure of a base station transmitter using a closed loop transmit diversity scheme according to the prior art. Referring to FIG. 6, an input bit stream is encoded by a channel encoder 601, and an output sequence of the channel encoder 601 is mapped into an M-ary symbol by an M-ary symbol modulator 602. The outputs of the symbol modulator 602 are provided to both Walsh cover parts 611 and 612. That is, the I-phase output of the modulator 602 is provided to both the Walsh cover parts 611 and 612, and the Q-phase output of the modulator 602 is also provided to both the Walsh cover parts 611 and 612. The Walsh cover parts 611 and 612 orthogonally-spread by multiplying their input complex symbols by a Walsh code allocated to the mobile station. Complex spreaders 621 and 622 complex-spread the outputs of their associated Walsh cover parts 611 and 612. A weight generator 651 generates weights C1 and C2 to be applied to the respective antennas, based on forward channel information fed back from the mobile station. Here, the feedback information can be either phase-related information or amplitude-related information. Complex multipliers 631 and 632 multiply the outputs of their associated complex spreaders 621 and 622 by the weights C1 and C2 provided from the weight generator 651, respectively. The outputs of the complex multipliers 631 and 632 are modulated into RF band signals by RF parts 641 and 642, respectively, and then radiated through first and second antennas ANT1 and ANT2.
In IS-2000 Release A for the cdma2000 system, a common pilot channel is transmitted through a first antenna ANT1, while a diversity pilot channel is transmitted through a second antenna ANT2. The mobile station calculates weight information for the two antennas using the common pilot channel and the diversity pilot channel, and then transmits the calculated weight information to the base station. Then, the weight generator 651 in the base station creates the weights C1 and C2 based on the received weight information.
Comparing theoretical maximum performance of the transmit antenna diversity schemes, the closed loop transmit antenna diversity scheme of FIG. 6 is superior to the open loop transmit antenna diversity scheme of FIG. 1 by 3 dB in terms of SNR (signal-to-noise ratio) required to attain a given bit error rate (BER). However, in the case of a non-ideal, normal Doppler channel, the closed loop transmit diversity scheme cannot properly perform the transmit diversity due to delay of the feedback information in a high-speed fading channel environment where the mobile station moves at high speed, so it has lower performance than the open loop transmit diversity scheme. That is, in the environment where the mobile station moves at high speed, it is never possible to obtain a gain of the closed loop transmit diversity. Therefore, there is a demand for a transmit antenna diversity method capable of obtaining a diversity gain over the whole speed range, regardless of the moving speed of the mobile station.
It is, therefore, an object of the present invention to provide a base station transmission apparatus and method for obtaining a transmit antenna diversity gain over the whole speed range, regardless of a moving speed of a mobile station in a CDMA mobile communication system.
It is another object of the present invention to provide a base station transmission apparatus and method for enabling the combined use of a closed loop antenna diversity scheme and an open loop antenna diversity scheme in a CDMA mobile communication system.
It is yet another object of the present invention to provide a base station transmission apparatus and method for obtaining a gain of a closed loop transmit antenna diversity scheme in a fading channel environment where a mobile station has a low speed, and obtaining a gain of an open loop transmit antenna diversity scheme in a channel environment where the mobile station has a high speed, in a CDMA mobile communication system.
According to a first object of the present invention, a base station transmission apparatus in a mobile communication system using transmit antenna diversity between a base station with a plurality of antennas and a mobile station, comprises a modulator for generating a complex symbol in response to a coded symbol; a first spreader for generating a plurality of different complex symbols in response to the complex symbol from the modulator, and generating a plurality of first spread complex symbols by spreading the plurality of the generated complex symbols with a first orthogonal code assigned to the mobile station; a second spreader for generating a plurality of same complex symbols being different from the plurality of the complex symbols in response to the complex symbol from the modulator, spreading the plurality of the same complex symbols with a second orthogonal code being different from the first orthogonal code, and generating a plurality of second spread complex symbols by multiplying the spread complex symbols by weights for the antennas, determined based on feedback information, received from the mobile station, indicating reception status of a base station signal; a summer for summing up the first complex symbols from the first spreader and the second complex symbols from the second spreader; and a transmitter for complex-spreading an output of the summer, shifting the complex-spread signals to a radio frequency band, and transmitting the shifted signals through the antennas.
According to a second object of the present invention, a base station transmission apparatus in a mobile communication system using transmit antenna diversity between a base station with a plurality of antennas and a mobile station, comprises a modulator for generating a complex symbol in response to a coded symbol; a serial-to-parallel converter for outputting two complex symbols with a reduced symbol rate by demultiplexing the complex symbol from the modulator; a first spreader for generating a plurality of different complex symbols in response to one complex symbol from the serial-to-parallel converter, and generating a plurality of first spread complex symbols by spreading the plurality of the complex symbols with a first sub-orthogonal code created from one orthogonal code assigned to the mobile station; a second spreader for generating a plurality of same complex symbols being different from the plurality of the complex symbols in response to another complex symbol from the serial-to-parallel converter, spreading the plurality of the same complex symbols with a second sub-orthogonal code being different from the first sub-orthogonal code, and generating a plurality of second spread complex symbols by multiplying the spread complex symbols by weights for the antennas, determined based on feedback information, received from the mobile station, indicating reception status of a base station signal; a summer for summing up the first complex symbols from the first spreader and the second complex symbols from the second spreader; and a transmitter for complex-spreading an output of the summer, shifting the complex-spread signals to a radio frequency band, and transmitting the shifted signals through the antennas.
According to a third object of the present invention, a base station transmission apparatus in a mobile communication system using transmit antenna diversity between a base station with a plurality of antennas and a mobile station, comprises a modulator for generating a complex symbol in response to a coded symbol; a diversity modulator for generating a plurality of different complex symbols in response to the complex symbol from the modulator; a Walsh cover part for generating a plurality of spread complex symbols by spreading the plurality of the complex symbols with an orthogonal code assigned to the mobile station; and a plurality of transmitters, a number of the transmitters being equal to a number of the complex symbols output from the Walsh cover part, for generating a plurality of complex symbols by multiplying one complex symbol from the Walsh cover part by weights for the antennas, determined based on feedback information, received from the mobile station, indicating reception status of a base station signal, complex-spreading the plurality of the complex symbols, shifting the complex-spread signals to a radio frequency band, and transmitting the shifted signals through the antennas associated with the weights.
According to a fourth object of the present invention, a base station transmission apparatus in a mobile communication system using transmit antenna diversity between a base station with a plurality of antennas and a mobile station, comprises a modulator for generating a complex symbol in response to a coded symbol; a diversity modulator for generating a plurality of different complex symbols in response to the complex symbol from the modulator; a spreader for generating a plurality of spread complex symbols by spreading the plurality of the complex symbols from the diversity modulator with an orthogonal code assigned to the mobile station; a switch for sequentially selecting the plurality of the complex symbols from the spreader in a given period; a complex multiplier for generating a plurality of weighted complex symbols by multiplying the complex symbol output from the switch by weights for the antennas, determined based on feedback information, received from the mobile station, indicating reception status of a base station signal; and a complex spreading and RF part for complex-spreading the plurality of the complex symbols from the complex multiplier, shifting the complex-spread signals to a radio frequency band, and transmitting the shifted signals through the antennas.